Acoustic surface waves travel across the surfaces of a single crystal material at a slower rate than acoustic bulk waves can travel through the crystal. For this reason, a greater delay time can be achieved in a smaller crystal by a surface wave than by a bulk wave.
For certain applications a good acoustic surface wave material should have a low acoustic velocity relative to other materials. It should also have large coupling strength, which is a measurement of the amount of energy required to generate a surface wave of a given energy.
Another property of a good acoustic surface wave material is a low optimum number of finger pairs. The optimum number of finger pairs (in the transducer) is the minimum number which can be used without the loss of energy (insertion loss) due to the generation of bulk waves in the crystal. Bulk waves would be received at the receiving transducer and would confuse the signal. Since the bandwidths of the generated and received signals are equal to the inverse of the number of finger pairs in the generating and receiving transducers, a small optimum number of finger pairs permits a wider selection of bandwidths.
Still another important property is a low temperature coefficient of delay. As the temperature of the crystal increases, the delay of the surface wave will either increase (a positive coefficient) or decrease (a negative coefficient). The delay of a surface wave in a good material should not be affected very much by temperature and therefore should have a temperature coefficient which is close to zero.
The following table gives the properties of the best-known acoustic surface wave materials:
__________________________________________________________________________ Temperature Coupling Optimum No. Effective Velocity Delay Coefficient of Strength of Finger Dielectric Material Orientation 10.sup.5 cm/sec .mu.sec/mm Delay ppm/.degree. C k.sup.2 (%) Pairs Constant __________________________________________________________________________ LiNbO.sub.3 YZ 3.40 0.29 + 93 4.8 4.5 50 Bi.sub.12 GeO.sub.20 (112),(111) 1.72 0.58 + 112 0.46 13 11 SiO.sub.2 YX 3.16 0.32 - 25 0.18 21 4.3 __________________________________________________________________________ Additional information about surface waves can be found in a January 1971 article in Vol. 9, No. 1 of Ultrasonics by John de Klerk titled "Ultrasonic Transducers 3. Surface Wave Transducers" and in a November 1972 article in Vol. 25, No. 11 Physics Today by John de Klerk titled "Elastic Surface Waves.